1. Field of the Invention
This invention relates to transparent x-ray intensifying screens and processes for making x-ray intensifying screens for use in radiography, and to fluorescent compositions comprising an isotropic phosphor transparent to x-rays and a polymeric binder.
2. Description Relative to the Prior Art
Transparent x-ray screens comprising alkali halide, alkaline earth halide, metal sulfide, and metal selenide phosphors have been prepared by various methods. These transparent screens have been shown to be desirable, because they make more efficient use of impinging x-ray radiation than thick conventional scattering screens, which "waste" a material amount of the radiation in diffusion of the light emitted near the back of the screen and internal absorption. Thick transparent screens, having a decreased number of reflections permit this light to reach the front surface of the screen with minimal deflection and to form a sharper image on the photographic film in contact with the screen. A greater proportion of the x-ray energy, absorbed by the phosphor and converted to light, is utilized in producing images without loss of sharpness.
Thin transparent screens, prepared by vapordeposition and containing only a phosphor, have also been made and exhibit lower speeds than scattering screens with equal phosphor coverage. Further, lacking a protective binder, these transparent screens are fragile and highly susceptible to physical damage. Thicker screens have been made by hot pressing but other defects in the manufacture of these large plates render them expensive to prepare.
U.S. Pat. No. 3,023,313, issued February 27, 1962 to De La Mater et al. discloses the use of a polymeric binder with a refractive index as close to that of an alkali metal halide phosphor as possible in order to produce x-ray intensifying screens with improved speed. However, because of substantial differences between the refractive index of selected binders and the refractive index of the phosphor, reflecting pigments must be added to the mixture to prevent "blurring of the image" and improve resolution. Thus, these screens are not truly transparent to light, and some decrease in utilization of absorbed x-rays is observed. The screens of De La Mater comprise a support preferably having a highly reflective base coating.
Swank, Applied Optics, 12, 1865-1870 (1973) describes the theoretical calculation of modulation transfer function (MTF), related to resolving power, of x-ray intensifying screens comprising transparent phosphors and a black backing. Swank discloses that although the MTF is enhanced when a black backing is used, 50% of the exposing radiation is absorbed by the backing. Thus, the speed of the x-ray intensifying screen is reduced.
Gasper, J. Opt. Soc. Am., 63, 714-720 (1973) describes the computation of theoretical efficiencies and MTFs of various screen-receiver systems, and reports that if a dark antihalation undercoat is applied to the back surface of a transparent screen, the MTF is only slightly improved. If, on the other hand, the back surface is made perfectly reflecting, there is degradation of MTF, but the efficiency of the screen is advantageously doubled, as is shown in FIG. 8 of Gasper.
Experimental verification of the Gasper calculations is provided by measuring the MTF of a transparent hot-pressed zinc sulfide screen coated with a dyed gelatin undercoat. Excellent agreement was found between the measured and computed MTFs. Gasper concludes that attempts to improve the MTF of a transparent screen result in an undesirable loss of efficiency. Given a choice between slight increases in MTF coupled with undesirable losses in efficiency (with an absorbing undercoat), and great increases in efficiency coupled with only slightly lower MTFs (reflective undercoat), the high efficiency screen with a reflective undercoat is clearly preferred by Gasper.
It is seen that transparent x-ray intensifying screens providing high resolution, while maintaining speed and efficiency, and which are resistant to physical damage and are easily and economically manufactured, are extremely desirable.